The present disclosure relates to the fabrication of high performance integrated circuits, and more particularly, relates to methods for reducing and monitoring of precipitated defects on the masking reticles used for the photolithography processes.
The manufacture of very large-scale integrated (VLSI) circuits requires the use of many photolithography process steps to define and create specific circuits on the semiconductor wafer (substrate) surface. A conventional photolithography system comprises of several basic subsystems such as a light source, optical transmission elements, transparent photo mask reticles, and electronic controllers. The system is used to project a specific circuit image, defined by the mask reticle, onto a semiconductor wafer coated with a light sensitive film (photoresist) coating. After image exposure, the film is then developed leaving the printed image of the circuit on the wafer. As VLSI technology advances to higher performance, circuits become geometrically smaller and more dense.
Advancements to photolithography processing were required. The implementation of very short wavelength (<300 nanometers) light in the deep ultraviolet (DUV) spectrum for use as the exposure source was required for successful printing of the smaller, high performance circuits. FIG. 1 illustrates a simple diagram of a conventional DUV photolithography system comprised of a DUV light source 102, mask reticle assembly 104, production wafer 106 and the system controllers 108.
A major issue with the shrinking geometries of the new technologies is precipitated defects. As device features shrink, the minimum size threshold for defects that either disturb or kill device/circuit performance shrinks as well. Photolithography systems are very much affected by such defects. Defects that fall or grow upon on the mask reticle may cause false, broken and defective images to be printed onto the semiconductor wafer. These defective images may render the entire circuit useless.
FIG. 2A illustrates a detailed view of a conventional mask reticle assembly 200. The reticle assembly comprises a mask reticle including a transparent reticle blank 202 with a thin opaque, metal film pattern 204 adhered to one side. This opaque pattern is the circuit image (reticle pattern) that is projected and exposed onto the production wafer. Mask reticle assembly 200 further includes a pellicle frame assembly 205 covers the reticle's metal pattern 204. This structure 205 features a transparent pellicle film 206, aligned parallel to the reticle pattern, with side support frames 208 that are attached to the outer edges of the reticle blank 202 via adhesive 210, to complete the enclosure covering the metal film pattern 204.
During the normal usage and handling of the mask reticle assembly 200 precipitated defects may collect and grow onto the assembly, both outside the reticle pattern 204 and pellicle frame 205, as well as inside the volume confined by the reticle blank 202 and pellicle frame. Certain defect accumulation may occur as a result of continued usage and handling of the assembly. Precipitated defects may be caused by air-borne contamination from the environment, pellicle glue, reticle pod outgassing, pellicle frame H2SO4 residue, from chemical growth and deposition from the reactions, and from the mixing of residual chemical fumes. To avoid high levels of product wafer defectivity and low yield, the mask reticle assembly 200 must be periodically disassembled and subjected to detailed time-consuming, expensive cleaning processes and/or replacements prior to reassembly for reuse.
Common chemicals used for cleaning the reticle's metal pattern are solutions based upon ammonia/ammonium, strong bases, and sulfates from strong (sulfuric) acids. Despite rigorous water rinsing after cleaning in the above solutions, minute residuals of the ammonia and sulfate ions may still remain on the reticle assembly components. The pellicle frame 205 enclosure around the metal pattern 204 is efficient for protecting the reticle from exterior precipitated defects. However, it is likely to trap the minute ammonia and sulfate residual ions inside the pellicle frame 205 near the reticle pattern 204. FIG. 2A illustrates these areas within and outside the reticle mask assembly 200. The confined volume 212 may contain higher concentrations of ammonia and/or sulfate ions than the areas 214 outside of the assembly 200. The high concentration of such chemical ions in the area 206 in such a close proximity to the reticle pattern 204 is not desirable. The mixture of such ions within the enclosed volume 212 and the slow inward diffusion of trace chemicals from the exterior environment through the pellicle frame 205 may slowly cause chemical precipitated defects to deposit and grow onto the reticle's metal pattern 204. For example, the reaction of ammonia and sulfate ions will produce ammonium sulfate precipitated defects.
It is also noted that ammonia/ammonium based chemicals and sulfuric acid are commonly used in other operations of IC fabrication. These other operations may be additional environmental sources for such contamination to migrate into the volume enclosed by the reticle blank 202 and pellicle frame 205. Furthermore, as the mask reticle is continuously used in the photolithography production, energy supplied from the light source may accelerate and promote precipitated defect growth. In time, the defects will grow to significant size and quantity, enough to cause defective printed patterns and low circuit yield. This phenomenon is illustrated in FIG. 2B. The reticle mask assembly 200 is shown with DUV light 216 exposure turned on. Precipitated defects 218 are shown growing and depositing upon or near the reticle pattern.
Although the pellicle frame 205 enclosing the reticle pattern 204 is not hermetically sealed, evacuation of the undesired chemical fumes is hard to control or can not be ascertained. Accordingly, it is desirable to have a controlled method for reducing the formation of the precipitated defects.